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C# Action Object (Lambda That Returns Void)

Understand the Action type. An Action is a delegate that is similar to a void method.
Action. This is a function object. Action objects return no values. The Action type is similar to a void method. This generic type is found in the System namespace.Void
To specify an Action, we must have no return value. The Action must never return a value onto the evaluation stack. Often we use lambdas to specify Actions.Lambda
First example. These Actions point to anonymous functions. These functions cannot return values. An Action instance can receive parameters, but cannot return values.Return

So: An Action instance is similar to a void method. The program shows how to Invoke the Action instances.

Action 1: This Action receives one int argument when invoked. It returns no result—it is a void delegate method.

Int, uint

Action 2: This Action receives 2 arguments, named "x" and "y." It prints them out to the console.

Action 3: This Action receives no parameters. So we have a method that receives no arguments, and returns no values.

C# program that uses Action type and Invoke using System; class Program { static void Main() { // Example Action instances. // ... First example uses one parameter. // ... Second example uses two parameters. // ... Third example uses no parameter. // ... None have results. Action<int> example1 = (int x) => Console.WriteLine("Write {0}", x); Action<int, int> example2 = (x, y) => Console.WriteLine("Write {0} and {1}", x, y); Action example3 = () => Console.WriteLine("Done"); // Call the anonymous methods. example1.Invoke(1); example2.Invoke(2, 3); example3.Invoke(); } } Output Write 1 Write 2 and 3 Done
Benchmark, methods. How much slower are delegate method calls than direct method calls? To test this, we use the Action type with a single parameter.

Overview: This program introduces the Method1 method, which contains some dummy code for testing.

Version 1: In the first loop, Method1 (which uses no Action) is called directly 100 million times.

Version 2: In the second loop, an Action instance that points to Method1 is invoked the same number of times.

Result: For a method with 1 parameter and no return value, the Action invocation costs more than 3 ns extra each time.

C# program that tests Action invocation using System; using System.Diagnostics; class Program { const int _max = 100000000; static void Main() { // Create Action delegate for Method1. Action<int> action = new Action<int>(Method1); var s1 = Stopwatch.StartNew(); // Version 1: use direct call. for (int i = 0; i < _max; i++) { Method1(5); } s1.Stop(); var s2 = Stopwatch.StartNew(); // Version 2: use Action (delegate) call. for (int i = 0; i < _max; i++) { action.Invoke(5); } s2.Stop(); Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000 * 1000) / _max).ToString("0.00 ns")); Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000 * 1000) / _max).ToString("0.00 ns")); Console.Read(); } static void Method1(int param) { // Dummy. if (param == -1) { throw new Exception(); } } } Output 0.32 ns Direct call 3.52 ns Action Invoke call
Benchmark, abstract. Next we benchmark abstract methods. An Action<int> can be an instance that points to any function that receives an int parameter.

Version 1: In this version of the code, we test an abstract class. We repeatedly call a method on it.

AbstractBenchmark

Version 2: Here we call a method on an Action instance. The code performs the same number of method calls as version 1.

Result: The Action was slower to call than the method from the abstract class reference.

C# program that benchmarks abstract using System; using System.Diagnostics; abstract class A { public abstract void MethodA(int y); } class AB : A { public override void MethodA(int y) { } } class Program { static void MethodA(int y) { } static void Main() { A abst = new AB(); abst.MethodA(0); Action<int> action = new Action<int>(MethodA); action.Invoke(0); const int max = 100000000; var s1 = Stopwatch.StartNew(); // Version 1: use abstract class method. for (int i = 0; i < max; i++) { abst.MethodA(i); abst.MethodA(i); } s1.Stop(); var s2 = Stopwatch.StartNew(); // Version 2: use Action. for (int i = 0; i < max; i++) { action.Invoke(i); action.Invoke(i); } s2.Stop(); Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / max).ToString("0.00 ns")); Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / max).ToString("0.00 ns")); Console.Read(); } } Output 3.54 ns Abstract call 6.68 ns Action Invoke() call
Dictionary. It is possible to use Action as the value in a Dictionary instance. This makes it possible to call functions by a string key.Dictionary

Here: In this example, we invoke two static void methods based on simple string keys.

Static

Idea: You could use a Dictionary of an abstract class. Then, instantiate each method as an instance of a derived class.

Inheritance
C# program that uses Dictionary with Action using System; using System.Collections.Generic; class Program { static void Main() { Dictionary<string, Action> dict = new Dictionary<string, Action>(); dict["cat"] = new Action(Cat); dict["dog"] = new Action(Dog); dict["cat"].Invoke(); dict["dog"].Invoke(); } static void Cat() { Console.WriteLine("CAT"); } static void Dog() { Console.WriteLine("DOG"); } } Output CAT DOG
Notes, Invoke. The Invoke method receives a number of arguments equal to the specific type of Action. We must specify these arguments in the Action type declaration.Generic Class, Method

Tip: This information is determined at compile-time, before runtime. If a program compiles, it uses the correct argument types.

Funcs. What is the difference between Action and Func? The Action type receives parameters but does not return a parameter. Func receives parameters and returns a result value.

So: An Action instance never returns anything, while the Func always returns something.

Func
A summary. Programs use Action for many purposes. The Action type specifies a function object that can receive parameters, but never returns a value onto the stack.

Note: Thanks to Turker Tunali for helping clarify the Action descriptive text. All contributions are appreciated.

Void methods. Action instances are void methods. As higher-order procedures, they can be passed around as objects. This is powerful.
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